This invention relates to optical communications and in particular to a method of modulating an optical carrier for use in a wavelength division multiplex (WDM) optical communications system.
In this specification the term “light” will be used in the sense that it is used generically in optical systems to mean not just visible light but also electromagnetic radiation having a wavelength between 800 nanometres (nm) and 3000 nm. Currently the principal optical communication wavelength bands are centered on 1300 nm, 1550 nm (C-Band) and 1590 nm (L-Band), with the latter bands receiving the majority of attention for commercial exploitation.
Exemplary WDM systems operating in the 1550 nm C-Band optical fibre communication band are located in the infrared spectrum with International Telecommunication Union (ITU) 200, 100 or 50 GHz channel spacing (the so called ITU Grid) spread between 191 THz and 197 THz.
With ongoing developments in optically amplified dense wavelength division multiplex (DWDM) optical links as the backbone of point-to-point information transmission and the simultaneous increase in bit rate applied to each wavelength and the simultaneous increase in the number of channels, the finite width of the erbium gain window of conventional erbium-doped optical amplifiers (EDFAs) could become a significant obstacle to further increases in capacity. Conventional EDFAs have a 35 nm gain bandwidth which corresponds to a spectral width of 4.4 THz. System demonstrations of several Tbit/s data rate are already a reality and the spectral efficiency, characterized by the value of bit/s/Hz transmitted, is becoming an important consideration. Currently, high-speed optical transmission mainly employs binary amplitude keying, using either non-return-to-zero (NRZ) or return-to-zero (RZ) signalling formats, in which data is transmitted in the form of optical pulses having a single symbol level.
In WDM several factors limit the minimum channel spacing for binary amplitude signaling, and in practice spectal efficiency is limited to ˜0.3 bits/Hz. Although increasing the per-channel bit rate tends to reduce system equipment, there are several problems that need to be overcome for transmission at bit rates above 10 Gbit/s; these being:                dispersion management of the optical fibre links, this becomes increasingly difficult with increased bit rate;        Polarisation mode dispersion (PMD) in the optical fibre causes increased signal degradation;        Realization of electronic components for multiplexing, de-multiplexing and modulator driving becomes increasingly difficult.        
One technique which has been proposed which allows an improvement of spectral efficiency is the use of quadrature phase shift keying (QPSK) [S. Yamazaki and K. Emura, (1990) “Feasibility study on QPSK optical heterodyne detection system”, J. Lightwave Technol., vol. 8, pp. 1646-1653]. In optical QPSK the phase of light generated by a transmitter laser is modulated either using a single phase modulator (PM) driven by a four-level electrical signal to generate phase shifts of 0, π/2, π or 3π/2 representative of the four data states, or using two concatenated phase modulators which generate phase shifts of 0 or π/2 and π or 3π/2 respectively. A particular disadvantage of QPSK is that demodulation requires, at the demodulator, a local laser which is optically phase-locked to the transmitter laser. Typically this requires a carrier phase recovery system. For a WDM system a phase-locked laser will be required for each wavelength channel. It further requires adaptive polarisation control which, in conjunction with a phase recovery system, represents a very high degree of complexity. Furthermore, systems that require a coherent local laser are sensitive to cross-phase modulation (XPM) in the optical fibre induced by the optical Kerr non-linearity, which severely restricts the application to high capacity DWDM transmission.
It has also been proposed to use differential binary phase shift keying (DBPSK) [M. Rohde et al (2000) “Robustness of DPSK direct detection transmission format in standard fibre WDM systems”, Electron Lett., vol. 36]. In DBPSK data is encoded in the form of phase transitions of 0 or π in which the phase value depends upon the phase of the carrier during the preceding symbol interval. A Mach-Zehnder interferometer with a delay in one arm equal to the symbol interval is used to demodulate the optical signal. Although DBPSK does not require a phase-locked laser at the receiver it does not provide any significant advantages compared to conventional amplitude NRZ signalling.
U.S. Pat. No. 6,271,950 discloses a differential phase shift keying optical transmission system, comprising a laser to generate an optical signal, a delay encoder to provide a different delay for each of M input channels and an M channel phase modulator which phase modulates the optical carrier signal with each of the differently delayed M input signal channels to form a time division multiplex (TDM) phase modulated optical signal.
However, in modern communication systems, the rate of development dictates that typically data streams multiply up by a factor of 4 every few years. At the time of application the proposed standard installation will use data streams of 10 Gbit/s and systems of 40 Gbit/s have been demonstrated. In addition to the matters discussed above, the practical problem then arises that new systems operating at high speeds have to co-operate with older systems.